The present invention pertains to propylene polymers, and more specifically to copolymers having both a low sealing temperature and a relatively high melting point. The present invention also pertains to a process for preparing said copolymers.
It is a known practice to use statistical propylene copolymers containing 1-butene and that can contain ethylene for the production of films.
Accordingly, patent publications EP-A-0145014 and EP-A-0146968, for example, describe the production of drawn films using terpolymers containing 8-30% molar of .alpha.-olefins having at least 4 carbon atoms and less than 3% molar of ethylene and which must contain at least 7% by weight of polymer insoluble in boiling heptane. These patent publications describe in particular copolymers which are produced by using a catalytic system containing a titanium trichloride-based solid, an alkylaluminum chloride and a methacrylic acid ester. The polymers, which contain relatively high quantities of chlorinated residues coming from the catalytic system, make it possible to prepare films having satisfactory optical and mechanical properties as well as advantageous sealing properties. According to these patent publications, films prepared by using polymers containing a smaller proportion of polymer insoluble in boiling heptane are not sufficiently resistant to scratching or have a low contact blocking ("anti-blocking" property). The example embodiments disclose binary propylene and butene copolymers and a terpolymer containing 0.9% by weight ethylene, 16% by weight butene, and a proportion of 15.8% by weight which is insoluble in boiling heptane, whose solubility in cold xylene amounts to 32.7% by weight.
Patent Publication EP-A-0577407 describes propylene copolymers containing ethylene and an .alpha.-olefin containing 4-10 carbon atoms. According to this patent publication, the proportion of polymer soluble in cold xylene must be sufficiently small for the polymer to present a satisfactory compromise between rigidity and sealing temperature. In this case, the patent publication recommends that for terpolymers containing less than 1.5% by weight ethylene, the proportion soluble in cold xylene not exceed 12% by weight.
It was noted that in certain cases the implementation of the polymers involves a number of problems. Specifically, it has been observed in the production of mono- or bi-drawn films that the polymers adhere to the drawing rollers.